Prior art suspension systems have been provided for motor vehicles to isolate the vehicle frame, or chassis, from impacts and vibrations resulting from vehicle wheels traversing uneven terrain. Vehicle ride characteristics have complex dynamics characterized by nonlinearities, vehicle roll and pitch, vehicle flexibility effects, varying parameters, unknown friction, deadzones and high amplitude disturbances. Excess vibration results in artificial vehicle speed limitations, reduced vehicle-frame life, biological effects on passengers and detrimental consequences to cargo. Present automobile suspension systems traditionally use passive suspension systems which can only offer a compromise between the two conflicting criteria of comfort and performance by providing spring and dampening coefficients of fixed rates. For example, sports cars usually have stiff, harsh, performance suspensions with poor ride quality, while luxury sedans typically have softer suspensions with poor road handling capabilities. Passive suspension systems have been provided by separate coil springs and shock absorbing dampers, in which power is input by a controlled power source to counteract impacts and vibrations resulting from traversing the rough terrain. The traditional engineering practice of designing spring and dampening functions as two separate functions has been a compromise from its inception in the late 1800s. As a result, vehicles have always been designed, styled and built around the space-weight requirements and performance limitations of traditional suspension configurations. Due to the demands of increased fuel mileage and decreased emissions, passenger and commercial vehicles are becoming lighter, which results in the differences between laden and unladen weights of the vehicles becoming so broad that traditional suspension systems are unable to span the load range effectively, causing serious degradation in performance of the vehicle ride quality, load handling and control.
Active suspension systems attempt to reduce these undesirable ride characteristics by providing active, powered components which isolate the car body from tire vibrations induced by uneven terrain, to provide improved comfort, road handling performance and safety for a variety of terrains and vehicle maneuvers. In active vehicle suspension systems, actuators are provided to actively apply forces which counteract and balance forces applied to the chassis of the motor vehicle. Such active systems have used various control schemes to determine the amount of force which actuators should apply to the vehicle chassis to provide a smoother ride, such as schemes based on balancing the forces acting on the chassis and schemes based on supporting the vehicle chassis at a selected ride height. Active suspension systems should be able to provide different behavioral characteristics dependent upon various road conditions, without going beyond the travel limits of active suspension components. However, active systems typically require large power inputs to provide an actuator that is quick enough to compensate for impacts and vibrations which occur at desired traveling velocities over rough terrain. The power requirements for such fully active suspension systems are generally prohibitive.
Some prior art passive suspension systems have utilized struts which contain a compressible fluid which is preferably a liquid, rather than a mixture of a liquid and a gaseous fluid. A rod extends into a cylinder and provides a fluid displacement member, such that the fluid pressure within the cylinder is increased by displacement of the compressible fluid when the rod is inserted further into the cylinder. A piston-like member is mounted to the inward end of the rod and provides a dampening device. Such struts have effectively combined into a single unit the spring and damper functions of prior art suspension system components. Control means have also been suggested for such strut systems, in which the fluid pressure within the struts are controlled to determine spring rate coefficients for the struts, such as the control system for vehicle suspension shown in U.S. Pat. No. 6,389,341, invented by Leo W. Davis, and issued May 14, 2002, which is hereby incorporated by reference as if fully set forth herein.
Although active control systems have been utilized to apply force to struts, that is, to add energy to the struts of suspension systems to prevent vehicle roll, they have not been combined with active dampening. In the prior art, dampening with active control systems has been accomplished by mechanical dampening systems in which the level of dampening is applied is a function of the velocity at which a dampening component is traveling relative to a cylinder of the strut. The level of dampening is not electronically controlled, but rather is accomplished via pressure applied to dampening members as a result of the velocity at which the dampening member is traveling through a fluid in the strut. For example, a dampening piston is disclosed in U.S. Pat. No. 6,389,341 having two sets of flow ports through the dampening piston, a first set of flow ports which are always open and a second set of flow ports through which fluid flow is controlled by a spring biased valve member. The valve member is moved by fluid pressure which is caused by the velocity of the dampening piston in traveling through a fluid, with increased fluid pressure due to higher velocities causing the valve member to move against a bias spring from a closed position to an open position to open the second set of flow ports through the dampening piston. This provides two set levels of dampening according to whether the valve member has been moved to the open position as a result of the velocity of the dampening piston traveling through the fluid.
Systems have been suggested for electronically controlling the level of dampening in suspension struts, primarily with use of electrorheological (“ER”) fluids or magnetorheological (“MR”) fluids. The viscosity of ER fluids is increased by application of an electric field to the fluid. Similarly, the viscosity of MR fluids are increased by application of a magnetic field to the MR fluid. The viscosity of ER fluids and MR fluids can be increased, such that field controlled valves to regulate and/or prevent fluid flow can be established by application of electric fields and magnetic fields, respectively, to flow passages. U.S. Pat. No. 5,985,168, invented by Pradeep P. Phule, and issued Nov. 16, 1999, discusses ER and Mr fluids, and discloses a magnetorheological fluid, and is hereby incorporated by reference, as if fully set forth herein.
Prior art suspension systems which use ER and MR fluids have encountered several problems which have prevented their use in struts. Namely, prior art ER and MR fluids have relied on the response of ferrous particles to applied electric fields and magnetic fields, respectively, to polarize the ferrous particles to increase the viscosity of ER and MR fluids. The ER and MR fluids have been provided by colloidal suspension of the ferrous particles in a base fluid. However, over relatively short periods of time, as compared to the typical service life of strut components for vehicle suspension systems, the ferrous particles have settled out of suspension in the base fluids rendering the respective electric field and magnetic field controlled valving schemes for determining the dampening characteristics of struts inoperable. Ferrous particle sediments have also damaged strut seals where strut rods pass from within strut cylinders to exterior suspension components.